The present invention relates to a signalling system. One aspect of the present invention relates to a free space point to multipoint signalling method and apparatus. Another aspect of the invention relates to a retro-reflector for use in a signalling system. According to a further aspect, the present invention relates to a multipoint to point signalling system.
There are a number of existing systems which allow point to multipoint communications. For example, there are existing radio and microwave broadcast systems in which messages are broadcast as radio/microwave signals from a central transmitter to a plurality of remote receivers. However, there are a number of disadvantages with this kind of broadcasting system. One disadvantage is that a large amount of the transmitted power is wasted due to the inevitable broadcasting of the signals into regions where there are no receivers. Another problem with this kind of broadcast system is that there is limited bandwidth (a few Megahertz for radio frequencies and up to one hundred Megahertz for microwave frequencies). Another problem with this kind of broadcast system is that it is subject to regulatory requirements. In particular, the transmission of signals in radio and microwave frequency bands is strictly controlled, with users being required to obtain a licence in order to broadcast.
Another type of known point to multipoint communications system employs cables (optical and/or electrical) and taps/junctions through which communications from a transmitter are passed to multiple receivers. However, these cable systems can be expensive, due to the cost of the cable itself and its installation, and is not suitable in some situations. In particular, although it may be practical to install cable along most streets, it may not be practical to install the cable from the street into every home and office along the side of the street. Cable systems are also not practical in temporary systems where a short term communications link is required.
Another type of known point to multipoint communication system is an optical broadcast system, in which the messages are broadcast as optical signals from an optical transmitter to a plurality of remote optical receivers. However, as with the radio and microwave broadcast systems, such an optical broadcast system suffers from the disadvantage that a large amount of the transmitted power is wasted due to the broadcasting of the signals into areas where there are no receivers. Additionally, since the power of the transmitted laser beam must meet eye-safety requirements, present systems can only broadcast over short distances of about a few metres.
According to one aspect, the present invention provides an alternative point to multipoint signalling system.
According to this aspect, the present invention provides a point to multipoint signalling system, comprising a first signalling device and a plurality of second signalling devices, wherein the first signalling device comprises: means for receiving signals transmitted from the plurality of second signalling devices, means for modulating the received signals with respective modulation data for the second signalling devices and means for reflecting the modulated signals back to the respective second signalling devices, and wherein each second signalling device comprises: means for generating a signal, means for transmitting the generated signal to said first signalling device, means for receiving the modulated signal which is reflected back from said first signalling device, and means for retrieving the modulation data from the reflected signal.
The present invention also relates to a retro-reflector for use in a signalling system. Optical retro-ref lectors are commonly used in a wide variety of applications from precision metrology to xe2x80x9ccats-eyexe2x80x9d road markers. The main function of the retro-reflector is to reflect incident light back towards the source. There are currently three different types of optical retro-reflectors commonly in use, namely the spherical reflector, the corner cube reflector and the plane mirror.
Plane mirrors are only effective as optical retro-reflecting elements when they are carefully aligned with the source, so that the reflective surface is orthogonal to the direction of travel of the light emitted from the source.
Corner cube reflectors typically comprise an arrangement of three mutually perpendicular mirrors. Alternatively, they can be constructed from a glass prism with three mutually perpendicular reflecting surfaces formed by the faces of the prism. Light incident on such a corner cube reflector is reflected from all three surfaces and travels back towards the source parallel to its original path. Typically these corner cube reflectors have acceptance angles (i.e. a field of view) of +/xe2x88x9230xc2x0, thereby eliminating the need for precise angular alignment with the source.
There are several different forms of spherical retro-reflectors. The most common comprises a relatively high reflective index sphere which focuses the incident light on the rear surface of the sphere (which can be coated with a reflective coating) where it is reflected back along its original path. This is the basis of the xe2x80x9ccats-eyexe2x80x9d type reflectors commonly used for road markings. Other types of spherical retro-reflectors comprise of one or more concentric spheres of differing refractive index or reflectivity, with the reflection occurring at the surface of the concentric spheres.
A second aspect of the present invention aims to provide an alternative type of retro-reflector.
According to this aspect, the present invention provides a retro-reflector comprising: a focusing member, having first and second focal planes, for focusing signals received from a source, a stop member, located substantially at said first focal plane, for blocking part of the received signals from said focusing member and a reflecting member, located substantially at said second focal plane, for reflecting said signal back to said source.
The second aspect of the invention also provides a retro-reflector comprising a telecentric lens for receiving and focusing light from a light source and a reflecting means located substantially at the focal plane of said telecentric lens for reflecting said light back to said light source.
A third aspect of the present invention aims to provide an alternative type of multipoint to point signalling system.
According to this third aspect, the present invention provides a multipoint to point signalling system, comprising a first signalling device and a plurality of second signalling devices located at preselected locations with respect to said first signalling device, wherein each second signalling device comprises: at least one sensor input terminal operable to receive a signal from a sensor and a light source operable to generate a light beam in dependence upon said sensor signal for transmission to said first signalling device and wherein said first signalling device comprises: a plurality of light detectors for detecting light transmitted from the plurality of second signalling devices and means for focusing the light transmitted from each second signalling device onto a respective light detector.